1. Field of the Invention
The present invention relates to a constant velocity universal joint used in power transmission in automobiles and various industrial machines, and more particularly to a fixed type constant velocity universal joint having eight balls.
2. Prior Art
Constant velocity universal joints are classified into a fixed type not sliding in the axial direction, and a slidable type. FIGS. 8a and 8b show a Rzeppa type constant velocity universal joint (hereinafter called BJ type) as a representative example of a fixed type constant velocity universal joint. This constant velocity universal joint comprises an outer ring 11 as an outer joint member having six arcuate guide grooves 11b extending in the axial direction in an inner spherical surface 11a, an inner ring 12 as an inner joint member having six arcuate guide grooves 12b extending in the axial direction in an outer spherical surface 12a, six balls 13 disposed between the guide grooves 11b of the outer ring 11 and guide grooves 12b of the inner ring 12, and a cage 14 for retaining the balls 13.
Centers P and Q of the guide grooves 11b and 12b are offset from the joint center O by an equal distance (PO=PQ) to the right and left side in the axial direction. That is, the center P of guide groove 11b of the outer ring 11 is offset from center O of the inner spherical surface 11a to the opening side of the outer ring 11 by distance PO. The center Q of guide groove 12b of the inner ring 12 is offset from center O of the outer spherical surface 12a to the inner side of the outer ring 11 by distance QO. The centers of inner spherical surface 11a of the outer ring 11 and outer spherical surface 12a of the inner ring 12 coincide with the joint center O.
One (not shown) of two shafts to be coupled is connected to the outer ring 11, and other (shaft part 15) is connected to the inner ring 12. Accordingly, the inner ring 12 has a tooth profile, i.e. serration or spline, 12c to be coupled with the shaft part 15. The outer ring 11 and inner ring 12 form a certain angle, and the balls 13 guided in the cage 14 are maintained within a plane perpendicular to a bisector of angle θ formed by the outer and inner ring 11 and 12, so that the constant velocity of the joint is assured.
In a fixed type constant velocity universal joint, thus far, a constant velocity universal joint applicable to high angle (maximum joint angle 50°) was an undercut-free joint (called UJ type) comprising six balls 13, but by increasing the number of balls and reducing the ball diameter, a more compact UJ type having eight balls with same strength and durability has been developed (for example, see Japanese Patent Application laid-open under No. H9-317784).